A transistor is a semiconductor device used to switch or amplify electronic or electrical signals. Early transistor usage was dominated by Bipolar Junction Transistor (BJT) devices. The BJT is a three terminal transistor constructed of doped semiconductor material, popular due to their ease of manufacture and speed. As demand for high-speed, low-cost, small-size digital components increased, Metal Oxide Field Effect Transistor (MOSFET) devices became more prevalent. The MOSFET typically includes a metal or polysilicon gate separated from a semiconductor region by an insulator. The semiconductor region generally includes a substrate of a first conductivity type, and a source region and drain region of a second different conductivity type located on either side of the semiconductor region, under the insulator. Two different MOSFET devices include an enhancement MOSFET and a normally “on” depletion MOSFET device.
The enhancement MOSFET includes a drain region and a source region isolated by the substrate. In the enhancement MOSFET, as voltage is applied to the gate, a channel forms on the surface of the semiconductor region between the drain and the source, allowing current to flow between the source and the drain.
In contrast, the depletion MOSFET includes a coupled source and drain region extending below the gate. Here, as voltage is applied to the gate, a depletion region forms under the insulator, narrowing the coupled region between the source and the drain, the narrowed region reducing the ability for current to flow between the source and the drain.